The role of microorganisms in coral health ... - Semantic Scholar

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The role of microorganisms in coral health, disease and evolution Eugene Rosenberg*, Omry Koren*, Leah Reshef*, Rotem Efrony* and Ilana Zilber-Rosenberg‡

Abstract | Coral microbiology is an emerging field, driven largely by a desire to understand, and ultimately prevent, the worldwide destruction of coral reefs. The mucus layer, skeleton and tissues of healthy corals all contain large populations of eukaryotic algae, bacteria and archaea. These microorganisms confer benefits to their host by various mechanisms, including photosynthesis, nitrogen fixation, the provision of nutrients and infection prevention. Conversely, in conditions of environmental stress, certain microorganisms cause coral bleaching and other diseases. Recent research indicates that corals can develop resistance to specific pathogens and adapt to higher environmental temperatures. To explain these findings the coral probiotic hypothesis proposes the occurrence of a dynamic relationship between symbiotic microorganisms and corals that selects for the coral holobiont that is best suited for the prevailing environmental conditions. Generalization of the coral probiotic hypothesis has led us to propose the hologenome theory of evolution. Scleractinian coral Scleractinian, stony or hard corals as they are often referred to, are animals that are responsible for building coral reefs.

Mutualistic interaction A close ecological relationship, between two (or more) species, from which both species benefit.

Symbiodinium (Gr. Symbion living together and Gr. dinos whirling). A genus of dinoflagellate algae. It is the dominant genus of algal symbiont in reef-building corals. *Departments of Molecular Microbiology and Biotechnology, Tel Aviv University, Ramat Aviv, Israel 69978. ‡ Teaching at the Open University of Israel, Raanana, Israel 43107. Correspondence to E.R. e-mail: [email protected] doi:10.1038/nrmicro1635 Published online 26 March 2007

Coral reefs are the largest structures made by living creatures. They contain an enormous diversity of organisms, comparable to rain forests1. The economic value of coral reefs has been estimated to be US$375 billion per year 2, largely derived from fishing, tourism and coastal protection activities. The building framework of reefs is provided by scleractinian corals (hard corals). Until recently, corals were considered to be the product of a mutualistic interaction between the coral and algae of the genus Symbiodinium, commonly referred to as zooxanthellae. However, recent research has demonstrated that corals also contain large, diverse and specific populations of other microorganisms3–9 that have apparently co-evolved with corals 6,10. The role of these microorganisms in coral health and disease has been the primary driving force for the emergence of the new field of coral microbiology. The aim of this article is to: first, review recent data on the abundance and diversity of microorganisms associated with healthy and diseased corals, with an emphasis on how environmental factors impact on the symbiotic process; and second, to suggest a new theory regarding the role of microorganisms in the evolution of corals and to extend the application of this theory to other organisms.

NATURE REVIEWS | MICROBIOLOGY

Microbiota of healthy corals Corals are composed of two layers of cells, the epidermis and gastrodermis, covered by a surface mucus layer and connected to a large, porous calcium carbonate skeleton (FIG. 1). These structures also interact with diverse forms of microbial life and the coral holobiont contains microbial representatives from all three kingdoms — Bacteria, Archaea and Eucarya as well as numerous viruses. Symbiodinium. In 1883 Karl Brandt reported that hard corals were associated with intracellular microscopic algae11, subsequently identified as dinoflagellates12. These algae were first cultured in the 1950s, resulting in the discovery of the new genus Symbiodinium. The apparent uniformity of the symbionts isolated from various hosts led to the assumption that all symbiotic dinoflagellates belonged to a single pandemic species Symbiodinium microadriaticum13. Molecular evidence has since demonstrated that the genus Symbiodinium is exceptionally diverse, containing multiple taxa14–17. Symbiodinium provide a large part of the energy requirements of their hosts by transferring photosynthetically fixed carbon to the coral18,19. Another lessappreciated function of algal photosynthesis in this system is the production of large amounts of molecular oxygen that allows for efficient respiration by the coral

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Seawater

Mucus layer Mouth

Bacteria (not to scale)

Epidermis Gastrodermal cavity Mesoglea

Gastrodermis Zooxanthellae

Calicoblastic epithelium Organic matrix CaCO3 skeleton

Figure 1 |The structure of coral tissue. The upper panel is a picture of the coral Oculina patagonica. Corals are composed of three structures that provide habitats for bacteria: the surface mucus layer, coral tissue (including the gastrodermal cavity) and the calcium carbonate (CaCO3) skeleton (see cartoon in lower panel). Zooxanthellae are present in the gastrodermis, shown in green in the figure.

Surface mucus layer A chemically complex viscoelastic gel layer that surrounds coral. Much of the mucus originates from zooxanthellae. It is secreted from epidermal mucus cells and subsequently modified by resident microorganisms.

Holobiont The host organism and all of its associated symbiotic microorganisms.

Oxygen radical An atom or group of atoms that have one or more unpaired electrons. A prominent feature of radicals is that they have extremely high chemical reactivity.

and associated prokaryotic microorganisms. In addition, the high concentration of oxygen (>200% saturation20) results in formation of oxygen radicals, which provide protection against infection21. A single species of coral can host multiple types of Symbiodinium21–25, often containing different taxa at different depths25. At present, it is not clear how other environmental factors, such as temperature, affect symbiont distribution. Bacteria. Corals provide three habitats for bacteria (FIG. 1): the surface mucus layer, coral tissue (including the gastrodermal cavity) and the calcium carbonate skeleton, each of which harbour a distinct bacterial population3,4. Moreover, it is likely that within each habitat there are microniches colonized by different bacterial species6. Initially, research in coral microbiology focused on the mucus layer of the coral structure, using traditional culturing techniques. These studies demonstrated that this layer supports a diverse and abundant beneficial bacterial community7,26–29, including nitrogen fixers27,30,31 and chitin decomposers26. The abundance of bacteria in

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the mucus layer has been estimated at 105–106 colonyforming units (cfu) per ml4,26. Interestingly, these values are approximately 0.2% of the total bacterial counts that were determined microscopically following SYBR gold staining4. As the local seawater environment that surrounds the coral structure yields viable and total bacterial counts of about 103 per ml and 106 per ml, respectively, the bacterial population of coral mucus is 100–1000-fold higher than that observed in seawater. The biochemical and microbiological properties of coral mucus have been reviewed recently32. Bacteria also colonize coral tissue and the amounts of culturable and total bacteria found in coral tissue are similar to those found in mucus4. However, those bacterial species that are abundant in coral tissue differ from the abundant species colonizing the surface mucus layer3,4. Coral skeletons are porous structures inhabited by a variety of bacteria. This endolithic community has been estimated to satisfy 50% of the total nitrogen needs of the coral33. Cyanobacteria in the skeleton of Oculina patagonica provide organic compounds (produced by photosynthesis) to the coral tissue34. These bacteria could be crucial for the survival of the coral when it loses its endosymbiotic algae35, a disease referred to as coral bleaching. Based primarily on culture-free methodology (BOX 1), a number of generalizations about the bacterial populations that are associated with coral structures are beginning to emerge. First, the diversity of bacterial species that are associated with a particular coral species is high, including many novel species. Second, the identity of the members of the coral bacterial community differs from the composition of the microbial community in the seawater surrounding the coral, suggesting the association between the coral and its microbiota is specific. Third, the species composition of the uncultured bacterial population is completely different from the cultured population. Fourth, similar bacterial populations, in terms of composition, are associated with the same coral species (even if geographically separated), whereas different populations are found on different coral species. Finally, as mentioned above, different bacterial populations are found in mucus, skeleton and tissue from the same coral fragment. Archaea. The presence of diverse and abundant populations of Archaea in hard corals has been demonstrated using culture-independent techniques9,36,37. Direct cell counts with Archaea-specific probes suggest that many corals contain >107 archaeal cells per cm2 of coral surface. However, recent evidence suggests that Archaea do not form specific associations with corals, as most archaeal sequences that were associated with coral also matched sequences that were found in the surrounding marine water column9. So far, little is known about the biological role of Archaea in the coral holobiont. Viruses. The study of coral viruses is still in its infancy, primarily because techniques for culturing these viruses have not been developed. However, both electron microscopy and SYBR gold staining have revealed the presence of

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Coral pathogens Over the past 30 years there has been an approximately 30% worldwide decline in the coral population, largely due to emerging diseases39. Out of more than twenty coral diseases described, the causative agent for only six of the diseases has been isolated and characterized (FIG. 2; TABLE 1). At the global scale, coral bleaching is the most serious disease threatening coral reefs. Coral bleaching is caused by the disruption of the symbiotic interaction between coral hosts and the endosymbiotic algae, Symbiodinium. The loss of the algae and/or its photosynthetic pigments causes the coral to lose colour (this is the bleaching

process). In general, coral bleaching coincides with the hottest period of the year40,41 and is most severe at times of warmer-than-normal conditions42. In two cases, coral bleaching was shown to result from a bacterial infection that occurs when seawater temperature is high — the wellstudied bleaching of O. patagonica in the Mediterranean Sea43,44 and bleaching of Pocillopora damicornis in the Indian Ocean and Red Sea45,46 (TABLE 1). Shortly after bleaching of O. patagonica was first reported in 1995 (REF. 47), it was demonstrated through the application of Koch’s postulates that the causative agent of the disease was Vibrio shiloi43,44. Furthermore, it was also established that the environmental factor that triggered the disease was an increase in seawater temperature48. Infection and the resulting bleaching phenotype only occurred during summer seawater temperatures (25–30°C) and not during winter temperatures (16–20°C). The mode of infection and the temperature dependence of each step in the process have been studied extensively49 and are summarized in FIG. 3. With the exception of the interactions between V. shiloi and O. patagonica, and Vibrio coralliilyticus and P. damicornis45,46, there has been only limited microbiological analysis of bleached corals. In one study, Ritchie et al.50 compared culturable heterotrophic bacteria from healthy and bleached Montastraea annularis in the Bahamas. Pseudomonas spp., which represented 13% of the total bacterial isolates obtained from disease-free corals, could not be isolated from bleached corals. By contrast, 30% of all bacteria isolated from bleached corals were Vibrio spp., a genus that was not present in healthy corals. Although the authors of this study suggested that a Vibrio spp. could have been responsible for bleaching, the observed replacement of the Pseudomonas spp. population with Vibrio spp. in the diseased coral could be either the cause or the result of the disease. To distinguish

a Oculina patagonica

b Gorgonia ventalina

c Acropora cervicornis

d Diploria strigosa

e Favia favius

f Acropora palmata

g Montastraea faveolata

h Diploria strigosa

Box 1 | Culture-free techniques Culture-free, DNA-based techniques were first applied to the study of coral bacteria by Rohwer et al.70 Although these techniques overcome the problem that < 1% of coral bacteria can presently be cultured, they have several short-comings that need to be considered: first, these techniques usually give no indication of total bacterial counts, only their relative abundances; second, there is bias in favour of some bacterial species over others, because of differences in the ease of DNA extraction71 and in the selection of PCR primers72,73; and third, the detailed biological characterization of bacterial species that are identified using these techniques can be delayed indefinitely as the conditions required for their in vitro culture are, by definition, unknown. Despite these drawbacks, PCR-derived technology has provided useful data on the structures of bacterial communities that are associated with different corals3–5,7,74–77.

numerous viruses in coral mucus and tissue. In addition, a recent report described the production of numerous virus-like particles by heat-shocked corals38.

SYBR gold staining A technique for counting bacteria and viruses in environmental samples. Particles that contain either DNA or RNA emit a bright and stable yellow-green fluorescence that can be enumerated by epifluorescence microscopy.

Endolithic community A group of organisms that live inside the pore space of rocks, in this case the space within the coral skeleton.

Coral bleaching The whitening of corals due to the loss of their symbiotic zooxanthellae or the pigments associated with the algae.

Koch’s postulates The four criteria designed to establish a casual relationship between an infecting microorganism and a disease.

Heterotrophic bacteria Microorganisms that use organic molecules as their main source of carbon and energy.

Figure 2 | Infectious diseases of coral. Photographs of the following diseases are shown: bleaching of Oculina patagonica (a); Aspergillosis of Gorgonia ventalina (b); white band disease of Acropora cervicornis (c); white plague disease of Diploria strigosa (d); white plague disease of Favia favius (e); white pox disease of Acropora palmata (f); yellow blotch disease of Montastraea faveolata (g); and black band disease of Diploria strigosa (h). We thank L. Richardson and NOAA for photographs b–d and f–h.



REVIEWS Table 1 | Coral pathogens Disease and location

Pathogen 43,44

Coral host

Comments

Bleaching in the eastern Mediterranean (FIG. 2a)

Vibrio shiloi

Oculina patagonica

Best-characterized example of a coral infectious disease

Bleaching and lysis in the Indian Ocean and Red Sea

Vibrio coralliilyticus45,46

Pocillopora damicornis

Bleaching occurs at 23–26°C; cell lysis occurs at higher temperatures

Aspergillosis in the Caribbean (FIG. 2b)

Aspergillus sydowii83,84

Gorgonians (sea fans)

Sources of the fungus are African dust storms and run-off from land85

White band in the Caribbean (FIG. 2c)

Vibrio carchariae86

Acropora spp.

V. carchariae is the presumed pathogen, but Koch’s postulates not demonstrated

White plague in the Caribbean (FIG. 2d)

Aurantimonas coralicida67

Several

A. coralicida is the first representative of a new genus

White plague in the Red Sea (FIG. 2e)

Thalassomonas loyana87,88

Several

Identity of pathogen was confirmed using phage therapy87

White pox in the Caribbean (FIG. 2f)

Serratia marcescens89

Acropora palmata

S. marcescens is a known pathogen of humans and domestic animals

Yellow blotch in the Caribbean (FIG. 2g)

Vibrio alginolyticus90 (+ 3 other Vibrio spp.)

Montastraea spp.

Temperature sensitivity of the disease is similar to bleaching

Black band (widespread)

Consortium91

Several

Bacteria produce sulphide, which kills coral tissue

(FIG. 2h)

between these two possibilities, infection experiments need to be carried out under controlled conditions.

Stress factors that contribute to coral disease Climate change51, water pollution52 and over-fishing53 are the three most frequently cited environmental stress factors responsible for the increase in the incidence of coral disease. Until recently, the evidence supporting these claims has been largely circumstantial, namely correlations between these stress factors and the frequency of disease. However, recent studies have provided direct experimental evidence demonstrating how each of these factors contributes to coral disease.

Alleopathy The harmful effect of one organism to another caused by the release of chemical compounds.

Climate change. The earth is undergoing accelerating climate change, driven by increasing concentrations of greenhouse gases. During the last century, the average global temperature increased 0.6 ± 0.2°C and it is predicted to increase by another 1.5–4.5°C this century54. All of the coral diseases shown in FIG. 2 occur more frequently and progress more rapidly in warm summer months. In the two documented examples of bacterial bleaching of corals46,49, the mechanisms by which temperature affects the process have been established. As noted above, expression of crucial bacterial virulence genes is temperature dependent. At warm temperatures, V. shiloi expresses a cell-surface adhesin that is required for bacterial adhesion to the coral surface55. At elevated temperatures, the microorganism also expresses Toxin P, which inhibits photosynthesis of the endosymbiotic algae56, and superoxide dismutase, which is required for survival inside the coral21. In the case of infection by V. coralliilyticus, synthesis of an extracellular proteinase, an important virulence factor, is temperature-dependent46.

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Water pollution. Elevated nutrients (for example, phosphate, nitrate, ammonia and dissolved organic carbon) in coastal waters have been suggested as a cause of reef decline. Field experiments using a slow release fertilizer in nylon bags demonstrated that increasing concentration levels of inorganic nitrogen and phosphate in the local coral environment increased the severity of aspergillosis and yellow blotch diseases57. A second study looked at the addition of small quantities of carbohydrates to corals in a flow-though system, which caused substantial coral mortality and an order of magnitude increase in the rate of growth of microorganisms in the coral mucus58. In the second study, it was suggested that mortality resulted from a disruption of the balance between the coral and its associated microbiota. These studies should be valuable in implementing coral monitoring and management plans. Over-fishing. Over-fishing reduces the number of fish that graze on algae, thereby increasing the concentration of algae that is present on coral reefs. Numerous studies have shown that various species of algae can influence corals negatively (reviewed by McCook et al. 59). Suggested mechanisms include alleopathy , smothering, shading, abrasion, overgrowth and harbouring potential pathogens. Recently, Smith et al.60 have demonstrated that algae can cause coral mortality by enhancing microbial activity through the release of dissolved compounds. Coral mortality could be completely prevented by the addition of antibiotics, supporting the hypothesis that the microbial community was the cause of coral death under conditions of abundant algal growth.


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c Penetration

d b

Multiplication and differentiation

Adhesion

Coral with zooxanthellae

g

h

a

Infection of corals by feeding worms

Chemotaxis

Uptake of V. shiloi by feeding fireworm

V. shiloi Temperature increase

e

Marine fireworm (winter reservoir and summer vector)

Temperature decrease Regaining of zooxanthellae

Toxin P production by V. shiloi

Loss of zooxanthellae (coral bleaching)

f Loss of V. shiloi

Loss of bacterial superoxide dismutase

Figure 3 | Infection of the coral Oculina patagonica by Vibrio shiloi. At high summer temperatures, motile V. shiloi are attracted to chemicals in the coral mucus (a)78, attach to the coral by a β-galactoside-containing receptor (b)55, and penetrate into the epidermal layer of the coral (c)79. After multiplying intracellularly, the bacterial density reaches 109 cells per cm3 and the bacteria differentiate into a viable-but-not-culturable (VBNC) state (d)79. At this stage, the pathogen expresses Toxin P (PYPVYAPPPWP), which inhibits photosynthesis of the intracellular algae80,56 leading to the loss of the algae and pigment (bleaching) (e). In the winter when the temperature drops (f), V. shiloi no longer produces superoxide dismutase (SOD)21 and is killed by coral-produced oxygen radicals81. The corals then regain their algae and pigmentation. Also at lower temperatures, V. shiloi does not produce Toxin P or the adhesin necessary to bind to the coral. An additional participant in the infection cycle is the marine fireworm, Hermodice carunculata82. During the summer, by feeding on the corals, the fireworms become colonized by V. shiloi and become a winter reservoir of the pathogen (g). During the spring and summer seasons, the fireworms can also function as vectors for the re-infection of the corals (h).

Stress resistance: the coral probiotic hypothesis Although corals contain an innate immunity system, they do not produce antibodies and are considered to lack an adaptive immunity system. However, recent data have demonstrated that corals can develop resistance to specific pathogens and adapt to higher temperatures. The coral probiotic hypothesis61 is proposed to explain these findings within the context of our current understanding of coral microbiology. Viable-but-not-culturable (VBNC) state When in this state, bacteria can no longer grow and form colonies on conventional culture media, but they show metabolic activity, maintain pathogenicity and, in some cases, return to active growth under appropriate conditions.

The innate immunity system. Like other invertebrates, corals possess innate or natural immunity. Basic coral defences have been reviewed by Mullen et al.62 and include physical barriers, such as the epidermis and mucus (the mucus surrounds the coral and is shed periodically removing trapped microorganisms), cellular components (phagocytic cells) that can engulf and destroy microorganisms on contact, and soluble


factors, including organic acids and antimicrobial products. Interestingly, a number of studies have shown that a high percentage of bacteria that colonize the mucus layer of hard corals produce antibiotics63–65, suggesting that coral-associated bacteria inhibit pathogen invasion and actively contribute to infectious disease resistance65. In addition, corals release potent bactericidal materials following mild mechanical stress66, a possible host defence system against infection following predator inflicted injury. Adaptation of corals to stress. From 1994 to 2002, bacterial bleaching of the coral O. patagonica in the eastern Mediterranean occurred every summer. During that time period, V. shiloi was repeatedly isolated from bleached corals and shown to cause bleaching of healthy corals in controlled aquaria experiments. However, since 2003 all attempts to isolate V. shiloi from bleached or healthy corals have been unsuccessful, and inoculation of fresh healthy corals (taken directly from the sea) with V. shiloi does not result in coral bleaching. It should be pointed out that bleaching of O. patagonica does still occur during the summer months, although the diseased coral characteristics have changed significantly. For example, at present, bleached corals reproduce sexually, whereas prior to 2002 they failed to produce eggs and sperm (Y. Loya, personal communication). The mechanism responsible for the current bleaching is unknown. In an attempt to understand the basis for the observed resistance of coral to V. shiloi, an infection experiment was carried out and compared with an experiment using ‘sensitive’ coral61 (FIG. 4). In the original experiment using ‘sensitive’ coral, V. shiloi adhered to the coral, penetrated into the epidermis and multiplied intracellularly. Using resistant coral, the infection pattern is different. The bacteria still adhere to the coral and penetrate into the tissues, but there is a decline in the levels of bacteria in the tissue after 24 hours and, by day four post- inoculation, V. shiloi can no longer be detected in the tissue. By some unknown mechanism, the coral can lyse the intracellular V. shiloi and avoid disease. There are other, less well documented, examples of coral communities becoming resistant to specific pathogens. For example, A. coralicida67, the bacterium responsible for the white plague disease outbreak on coral reefs off the Florida Keys in 1995, can no longer infect these corals68. In a study of bleaching patterns in reef corals, Brown et al.69 came to the conclusion that variation in bleaching susceptibility can arise from ‘experience-mediated tolerance’. Their data demonstrate that corals that were exposed to high solar radiation for 2–3 months, prior to high temperature, were more resistant to bleaching than corals that were exposed to lower radiation. Since no change could be found in the intracellular algae during the adaptation period, the authors offered no explanation for their interesting finding. In the next section, we discuss a hypothesis that provides an explanation for experience-mediated tolerance and other phenomena described above.


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Intracellular V. shiloi (cell per cm2)

1010

109

108

107

106 0

1

2

3

4

5

Days after infection

Figure 4 | Coral resistance to Vibrio shiloi infection. Following inoculation of O. patagonica — collected prior to 2003 — V. shiloi adheres to the coral, invades the epidermis and replicates intracellularly (shown in purple). In a similar experiment, using O. patagonica collected since 2003, the bacteria can still adhere and penetrate coral tissues; however, at 1 day post-infection, there is a decline in the intracellular levels of V. shiloi and the pathogen is no longer detectable (106 per cm2) after 4 days (shown in orange)60.

The coral probiotic hypothesis. The ability of coral to adapt to environmental stresses, including elevated temperature conditions and infection by specific pathogens, has led to the development of the coral

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probiotic hypothesis 61. This hypothesis posits that a dynamic relationship exists between symbiotic microorganisms and corals at different environmental conditions that selects for the most advantageous coral holobiont in the context of the prevailing conditions. By altering the structure of its resident microbial community, the holobiont can adapt to changing environmental conditions more rapidly and with greater versatility than a process that is dependent on genetic mutation and selection of the coral host. The following data discussed in this article, and presented previously61, support the coral probiotic hypothesis: first, corals contain a large and diverse microbial population associated with their mucus and tissues; second, coral-associated microorganisms can benefit their host by various different mechanisms, including photosynthesis, nitrogen fixation and the production of antimicrobials; third, the coral-associated microbial population undergoes rapid changes when environmental conditions are altered; and fourth, although lacking an adaptive immune system, corals can develop resistance to pathogens. The coral probiotic hypothesis could help explain the evolutionary success of corals and moderate predictions of their demise. Many of the arguments presented to support this hypothesis of adaptation and evolution are relevant to other invertebrates, and to higher animals and plants. Indeed, extrapolating from this hypothesis leads us to propose a higher order of postulation, namely, the hologenome theory of evolution (BOX 2).

Box 2 | A proposal — the hologenome theory of evolution

Hologenome The combined genomes of the holobiont.

Commensalism A symbiosis in which one organism is benefited and the other is neither benefited nor harmed.

The theory is based on the following well-established empirical data: • All animals and plants establish symbiotic relationships with microorganisms. Often the number of microorganisms and their combined genetic information far exceeds that of their host cells. We refer to the host and its symbiont population as the holobiont, and the host genome and the genomes of all the symbiotic microorganisms as the hologenome. • Different host species contain different symbiont populations and individuals of the same species can also contain different symbiont populations. Therefore, genotypic and phenotypic variation exists between hosts of the same species and between their microbiota. • The association between a host organism and its microbial community affect both the host and its microbiota; the nature of the interaction can range from mutualism through commensalism to that of a pathogenic interaction. • The genetic information encoded by microorganisms can change under environmental demands more rapidly, and by more processes, than the genetic information encoded by the host organism. There are at least three mechanisms by which the genetic information encoded by a microbial population that is symbiotically associated with a host can change: first, by alterations in the relative abundance of microorganisms currently associated with the host; second, through the introduction of new microorganisms from the environment; and third, by genetic alteration of the existing microbial population through mutation, horizontal gene transfer and subsequent selection. Each of these mechanisms is more versatile and can occur in a much shorter timeframe than the alteration and selection processes required for host genome evolution. In summary, the genome of the host can act in consortium with the genomes of the associated symbiotic microorganisms to create a hologenome. This hologenome — given the diversity and fast growth rates of microorganisms — can change more rapidly than the host genome alone, thereby conferring greater adaptive potential to the combined holobiont organism. Each of these points taken together lead us to propose a hologenome theory of evolution: the holobiont with its hologenome should be considered as the unit of natural selection in evolution, and microbial symbionts have an important role in adaptation and evolution of higher organisms. Therefore, microorganisms are essential not only in the health and disease of individual higher organisms, but they also are a significant factor in species survival and evolution. This hologenome theory of evolution is derived primarily from an understanding of the biology of corals. However, a large body of data exists in the literature relating to many eukaryotic organisms and their interaction with symbiotic microorganisms — a literature that could be re-evaluated in light of this theory.

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REVIEWS Conclusions and future research During the past decade there has been substantial progress made in determining the abundance and diversity of microorganisms that are associated with healthy and diseased corals. These microorganisms have an important role in the nutrition and disease-resistance of healthy corals. When the corals are exposed to environmental stress, the microbial population undergoes a change, which can lead either to adaptation to the new condition or to coral disease. The coral probiotic hypothesis incorporates the role of microorganisms within the adaptation and evolution of corals, and the hologenome theory of evolution generalizes on the hypothesis to include the effect of microorganisms on all animals and plants.

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Coral microbiology is still in its infancy and promises to be a fruitful area of future research. The fundamental interactions between microorganisms and their coral hosts remain to be elucidated. The causative agents of only a few diseases have been established and little is known about modes of transmission and disease reservoirs. Furthermore, the biology of coral viruses has yet to be explored even at the most rudimentary level. Understanding the complex interactions between microorganisms and their coral hosts will provide the necessary data to evaluate both the coral probiotic hypothesis and the hologenome theory of evolution. Finally, and most importantly, these studies will undoubtedly help decision makers take the right steps to conserve coral reefs for the future.

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Acknowledgements This research was supported by funds provided by the Israel Center for the Study of Emerging Diseases and the Coral Reef Targeted Research Program (CRTR). The CRTR is a partnership between the Global Environment Facility, the World Bank, the University of Queensland, NOAA and research institutions.

Competing interests statement The authors declare no competing financial interests.

FURTHER INFORMATION Eugene Rosenberg’s homepage: http://tau.ac.il/lifesci/ departments/biotech/members/rosenberg.html CORIS major reef-building coral diseases homepage: http://www.coris.noaa.gov/about/diseases/ CRC reef research centre homepage: http://www.reef.crc. org.au/discover/coralreefs/Coraldisease.htm Global coral disease database: http://www.unep-wcmc.org/ marine/coraldis/home.htm Khaled bin sultan living oceans foundation web site: http:// www.livingoceansfoundation.org/index.php NOAA coral health and monitoring program: http://www. coral.noaa.gov/coral_disease Reefbase web site: http://www.reefbase.org/ Reef relief web site: http://www.reefrelief.org/Image_ archive/diseases/index.shtml The coral disease homepage: http://ourworld.compuserve. com/homepages/mccarty_and_peters/Coraldis.htm Access to this links box is available online.
